The present invention relates to a process for preparing saturated branched chain fatty acids or alkyl esters thereof involving subjecting unsaturated fatty acids having 10 to 25 carbon atoms, alkyl esters thereof or mixtures thereof to a skeletal isomerization reaction in the presence of water or a lower alcohol at a temperature of about 240° C. to about 280° C. using a combination of a stericly hindered Lewis base and zeolite as a Brönsted or Lewis acid catalyst, and isolating saturated branched chain fatty acids or alkyl esters thereof or mixtures thereof from the reaction mixture obtained by the skeletal isomerization reaction. The yield of said saturated branched chain fatty acids is ≧70 wt %. The stericly hindered Lewis base is a tertiary amine or phosphine with linear or branched C1 to C6 alkyl or phenyl groups attached thereto.
Environmental concerns over the use of petroleum-based materials in the lubricant industry have stimulated much research to find suitable alternatives. In this regard, lubricating fluids derived from renewable fats and oils are of interest because of their purported advantages over petroleum-based materials (Hill, K., Pure Appl. Chem., 79: 1999-2011 (2007)). Among the cited advantages of fatty acid derived lubricants are their lower toxicity, lower flammability since they have lower vapor pressures, and better biodegradability compared to petroleum-based materials. Potential applications for such bio-based fluids can range from lubricants, greases, additives, polymers, organic chemicals and more. In fact, there are many commercial products in the market that are derived from renewable resources. For instance, polylactide polymers and 1,3-propanediol, important intermediates for polymer syntheses, are derived from biomass sugars by fermentation and are cost competitive with petroleum-based materials (Carole, T. M., et al., Applied Biochem. and Biotech., 113-116: 871-885 (2004)).
Vegetable oils also are promising candidates as replacements for petroleum-based materials since they have excellent lubricity properties (Swern, D., Baily's Industrial Oil and Fat Products, Third Edition, John Wiley & Sons, New York). Although these oils themselves have some commercial use, it is limited due to the presence of double bonds within their fatty acid alkyl chains which lead to oxidative stability problems when used at high temperature. Over the past decades, numerous chemical methods including electrophilic, nucleophilic, oxidative, and metal-catalyzed reactions have been developed that convert the common fatty acids found in natural fats and oils to novel oleochemical compounds that have improved and/or new properties over the starting fatty acids. For example, chemical processes for the modification of soy oil for use in greases, hydraulic and drilling fluids, and printing inks have been developed (Erhan, S. Z. and M. O. Bagby, J. Am. Oil Chem. Soc., 68(9): 635-638 (1991); Erhan, S. Z., et al., J. Am. Oil Chem. Soc., 69(3): 251-256 (1992); U.S. Pat. No. 5,713,990).
Saturated branched-chain fatty acid isomers (sbc-FAs), commonly referred to as isostearic acids, are derived from unsaturated fats and oils as a mixture of mono-methyl branched fatty acids (2, FIG. 1). Such mixtures of fatty acids are of commercial interest because they are liquid at low-temperatures, have good lubricity properties, and have good oxidative stabilities because of their lack of double bonds. Isostearic acid type products are currently used in the formulation of cosmetics, body washes, lubricants and fuel additives, surfactants, soaps, and coatings. Approximately 100 million pounds of these acids are consumed globally each year. Currently, the bulk of sbc-FAs 2 are obtained as coproducts from reactions that predominantly produce dimer fatty acids (6, FIG. 1). The typical yields of sbc-FAs 2 are 25-50 wt %, and their isolation and purification from the dimer acid 6 are labor-intensive. New processes that give higher yields and higher selectivity of sbc-FAs at a lower cost and with improved ease of isolation from other coproducts would be highly advantageous; such processes would expand their present use and/or open new outlets for these type of fatty acids.
We have developed a more efficient and economical process that maximizes sbc-FAs production and minimizes the bimolecular reactions that produce dimer products 6 as well as other unwanted coproducts (stearic 3, hydroxystearic 4, and γ-stearolactone 5; FIG. 1). We have also improved catalyst stability for multiple reuses.